The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
Semiconductor memory may be provided within a semiconductor package, which protects the semiconductor memory and enables communication between the memory and a host device. Examples of semiconductor packages include system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted and interconnected on a small footprint substrate. A number of semiconductor die may be mounted in an offset stack so that the die bond pads on each die are left exposed at an edge of the die. The die bond pads of the respective die in the stack may then typically be wire bonded to each other and the substrate to allow signal exchange to/from select die in the die stack.
It is often desirable to make one or more of the die bond pads on a die redundant. For example, the die bond pads used for the chip-enable signal may have a main die bond pad and redundant, or optional, die bond pad that are electrically connected to the same contact pad on the substrate.
There are two traditional methods for wire bonding to a main and optional die bond pad. The first is a so-called “Y”-shape wire bond. Using a simple example of a first die D1 mounted on a substrate and a second die D2 mounted on die D1, it may be desirable to make first and second redundant die bond pads on the second die D2. A wire bond may be formed from the substrate to a first die bond pad on the D1, and a second wire bond is formed straight up from the first die bond pad on D1 to a first die bond pad on D2. Then, to form the redundant “Y”-shape bond, a third wire bond is also formed diagonally from the first die bond pad on D1 to a second die bond pad on D2 (thus forming a “Y”-shaped bond involving the first die bond pad on D1 and the first and second die bond pads on D2). This traditional bonding method is problematic in that three wire bonds are formed on the first die bond pad on D1 (one from the substrate and two to die D2). Three bonds on the same bond pad presents a reliability risk such as electrical shorting or failure (where a bond may separate from the pad).
A second traditional method for redundant wire bonding is to form the redundant wire bond to die D2 directly from the substrate. A wire bond may be formed from a first contact pad on the substrate to a first die bond pad on the D1, and second wire bond is formed straight up from the first die bond pad on D1 to a first die bond pad on D2. Then, to form the redundant wire bond, a third wire bond is formed diagonally from the first contact pad on the substrate to a second die bond pad on D2 (passing over die D1). In this simple example, the third wire bond goes from the substrate to the second die but semiconductor devices would more commonly have larger numbers of die (4, 8, 16 or even 32 die). Forming a wire bond directly from the substrate to an upper level die results in a long wire bond presenting a reliability risk such as shorting against other wire bonds or collapse of the long wire bond.